1. Field of the Invention
The present invention relates to improvements in solvated mesophase pitch. More specifically, the current invention provides a solvent system suitable for use as the solvating component of high melting or unmeltable mesophase pitches. Additionally, the current invention provides a solvent system suitable for producing a high molecular weight mesophase pitch.
2. Prior Art
Mesophase pitches are carbonaceous materials which contain mesophases exhibiting optical anisotropy due to an agglomerated layered structure. The molecules have aromatic structures which through interaction are associated together to form ordered liquid crystals which are either liquid or solid depending on temperature. Mesophase pitch is not ordinarily available in existing hydrocarbon fractions obtained from normal refining procedures. Mesophase pitch, however, can be prepared by treatment of aromatic feedstocks which is well known in the art. In known processes, a growth reaction converts relatively small aromatic molecules into larger mesophase-size molecules and these molecules are concentrated. Thus, mesophase is extracted from pitch by treatment of aromatic feedstocks.
It is known that mesophase pitches can be drawn into pitch based carbon fibers which have numerous commercial uses. A challenge in preparing a high-performance carbon fiber from a mesophase pitch resides in the fact that a significantly high temperature is necessary to use at the spinning stage because of the high softening point of the pitch.
The present invention is a product of ongoing research in the field of solvated mesophase pitch. Solvated mesophase pitches were disclosed as early as U.S. Pat. No. 5,259,947 (owned by the Assignee herein) which is incorporated herein by reference. The solvated mesophase contains a small percentage by weight of solvent in the liquid crystalline structure so that it melts or fuses at a lower temperature. As noted, in the ""947 Patent and subsequent patents relating to this subject matter, solvated mesophase pitch has several advantages over traditional mesophase pitch. A primary advantage is the ability to use high melting or unmeltable mesophase pitch in carbon fiber spinning processes.
Prior to the current invention, the principal solvents used as the solvating component consisted of 1 to 3 ring aromatic compounds. The aromatics are a series of hydrocarbon ring compounds. While these 1 to 3 ring compounds are effective, they provide only a limited range of compatibility with heavy aromatic pitches.
In some applications, it is advantageous to have higher boiling point solvating solvents. This allows processing of the melted pitches at ordinary (in other words, atmospheric) pressure.
It is additionally advantageous to have higher boiling point solvating solvents which extend to higher temperatures. This will extend the range over which solvent evaporation rates are controlled when making or processing pitch artifacts.
It is, therefor, a principal object and purpose of the present invention to produce new solvents which makes processing of the carbon pitches more facile.
It is a further additional object and purpose of the present invention to produce a new solvent or solvating agent which solvates especially high melting mesogens.
It is a further object and purpose of the present invention to produce a novel solvent which promotes increased fiber attenuation during spinning.
It is a further object and purpose of the present invention to provide a high boiling point aromatic solvent as a useful component in extracting solvents in order to isolate heavy aromatic pitches from isotropic or mesophase pitches.
It is a further object and purpose of the present invention to isolate mesogenic insolubles by solvent fractionation.
The current invention provides a solvent system suitable for use as the solvating component of a solvated mesophase pitch. The solvent system comprises a mixture of aromatic hydrocarbons having boiling points in the atmospheric equivalent boiling point (xe2x80x9cAEBPxe2x80x9d) range of about 285xc2x0 to about 500xc2x0 C. (about 550xc2x0-932xc2x0 F.). In the solvent system, at least 80% of the carbon atoms are aromatic as characterized by carbon 13 NMR.
The aromatic hydrocarbon compounds making up the solvent system are selected from the group consisting of (i) aromatic compounds and N, O and S heteroaromatic compounds having 2 to 5 aromatic rings, (ii) substituted aromatic compounds and N, O and S heteroaromatic compounds having 2 to 5 aromatic rings wherein said substituents are alkyl groups having 1 to 3 carbons (C1 to C3), (iii) hydroaromatic compounds and N, O and S heteroaromatic compounds having 2 to 5 aromatic rings, (iv) substituted hydroaromatic compounds and N, O and S heteroaromatic compounds having 2 to 5 rings wherein said substituents are alkyl groups having 1 to 3 carbons, and (v) mixtures thereof. Additionally the aromatic hydrocarbon compounds can contain up to ten weight percent (10%) heteroatoms of nitrogen, oxygen and sulfur. When present, the heteroatoms predominately occur in stable aromatic ring structures such as pyrroles, pyridines, furans and thiophenes. The new solvents proposed herein facilitate the handling and use of solvated mesophase pitch.
The current invention additionally provides a solvent system for extracting isotropic and mesophase pitches. The solvent system suitable for extracting the pitches comprises a first solvent system as described above for solvating a mesophase pitch in combination with a second aromatic solvent system comprising 1 to 3 ring aromatic compounds having a solubility parameter in the range of 8 to 11.5 wherein said substituents are alkyl groups having 1 to 3 carbons, and mixtures thereof. The ratio of the first solvent system to the second solvent system may range from about 1:20 to about 2:5.
The extraction solution is added to a pitch in a solution to pitch ratio ranging from about 3:1 to about 20:1. The pitch is then extracted to yield a mesogen residue. Using the inventive solvent system, one achieves excellent control of the extraction process. Additionally, any residual solvent in the mesogen product is a suitable solvent for forming a solvated mesophase pitch.